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Community and Q&A

Root cellar ceiling: rigid foam install

Kevin F | Posted in Green Building Techniques on

I’ve seen many pieces of advice on here on fastening rigid foam insulation to basement walls, etc., but I have a somewhat unique fastening situation I’m trying to sort out.

Attached to the basement of our new house we built a root cellar. Basically it is a 3-sided concrete bunker with concrete walls and ceiling and an open (gravel) floor. The outside of the exterior walls are backfilled to about the top of the wall (8′ walls) and then there is a 6″ reinforced concrete slab poured over the top. The inside wall is the basement wall of the house and is insulated on the basement inside (and yes, everything is carefully air sealed between the basement and the wall.

The roof slab also serves as the floor for an attached covered porch that covers the entirety of the slab. However, like any open porch, it does get wet at times in a big rain. The whole thing is about 6′ x 20′.

The goal of this bunker is to be used as a root cellar. Last year, in a fairly mild winter (climate zone 6) the porch had not been built yet and snow above was sufficient to keep the space a nice even 38 degrees or so all winter (free refrigeration is pretty awesome!). However, without two feet of snow on top and in a cold winter, it will certainly freeze without additional insulation from the uninsulated ceiling, and possibly some wall insulation.

So, my questions are: how much insulation to people think is needed on the ceiling to keep the space from freezing, as well as keep it somewhat cool during the summer? And do I need to extend some insulation down the sides of the wall to address the thermal bridge of the concrete walls?

I have a bunch of scrap 2″ XPS I am hoping to use. Assuming people think that 2″ XPS is sufficient, what is the best way to attach this to the ceiling? The concrete is 4,000 psi and probably won’t be terribly fun to sink fasteners in, but I feel like on a ceiling just foam glue is not going to cut it like it might on a basement wall.

Lastly, given that the top of the slab will get wet with some frequency, should I be applying some sort of sealer to the underside before installing foam?

Thanks for all your good advice!

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  1. GBA Editor
    Martin Holladay | | #1

    Whenever I hear about this type of concrete roof exposed to rain, I advise people to remember to include roofing. A concrete slab is not a roof.

    In your case, it may be too late. But ideally, you would install some type of roofing (for example, EPDM) above the slab before installing your patio floor.

    You can install your interior rigid foam with fender washers and TapCon fasterners or with Plasti-Grip PMF fasteners manufactured by Rodenhouse, Inc.

    If you want to experiment, you might want to let your cellar experience two or three winters without any ceiling insulation. If your cellar is relatively airtight, it might not freeze, even without any ceiling insulation.

  2. Kevin F | | #2

    Thanks Martin. I may not have been clear in my original post - there is a roof above the concrete slab it's just a porch roof that is open on three sides. So it mostly stays dry unless there's a good wind.

    Unless you're suggesting that an EPDM membrane be installed over the concrete even in that situation, in which case that is still theoretically an option as there is no floor above it (we weren't planning on one).

  3. GBA Editor
    Martin Holladay | | #3

    I'm a roofer, so I would install EPDM over this slab. (You're the one who noted that "the top of the slab will get wet with some frequency.") Ultimately, this is a judgment call.

  4. Jon Harrod | | #4

    Snow has an R value of about 1 per inch. So, if the snow pack last winter was yielding the desired temperature balance, you could try to mimic that with an equivalent amount of XPS at R~5 per inch.

  5. PAHunter62 | | #5


    I recently built a new home and basically did the exact same thing you did under my back porch. I have a 10x20 area divided into two 10x10 rooms under the porch, one with a gravel floor and one with a cement floor. Each room has two 4" ports to the outside where I plan on bringing cold air in and exhausting the warm air out.

    I have not done any insulation yet and am only able to get the gravel floor room to about 54 degrees. The poured concrete walls are acting as a heat radiator, so I need to address that next. How thick (and what kind) of insulation did you use on the inside walls? How are you ducting/managing your air transfer? Natural convection, or using fans? Very interested as I have almost the exact setup and want to control in the 34-40 degree range through the late fall/winter months.

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